21 Nov
2004
21 Nov
'04
4:59 a.m.
On Sun, 2004-11-21 at 01:42 +0000, Tony Duell wrote:
It appears to be controlling the gate of an SCR, the anode of which goes
to the live input via a thermal resistor (which is placed next to that
5W resistor which keeps failing), and the cathode of which goes to the
'hot' ground rail. Some form of overload protection, I assume.
Odd.. I wonder how the voltage feedback works, then. It's _possible_ that
the chopper control circuit is on the isolated side of the PSU (and
therefore no opto-isolator on the voltage feedback loop),
transformer-coupled to the chopper transistors (DEC were fond of doing
this), but I wouldn't assume that without checking
That appears to be exactly right. The gates for the 8 chopper
transistors link into a small multi-tap transformer. I've not
investigated what's on the primary side of that yet. The 'outputs' from
the chopper transistors driving the switching transformer also hook back
into this small transformer too though via a pair of taps, so I'm not
sure what that's about (yet).
Maybe OK. It could just be the charging current of the smoothing caps
(they'll cahrage a lot more slowly with the lamp in series).
I've taken the bridge out of circuit just to be sure and that still
checks out on a meter. I've borrowed a pair of monster 5000mfd caps out
of a known-good PSU so I can rule out probs with the NCR's caps too.
it is? I
suspect the SOC chip is an opto-isolator (it only has 6 pins
and is curiously in a white package). Not sure about the SG chip but I
Almost certainly an optoisolator. Probably for the voltage feedback loop
(LED on the secondary side of the supply, connected to some kind of
voltage sense circuit, transistor connected to the chopper control side
of things). The circuit you've found sounds like something
designed to blow the fuse
(or at least shut the supply down) if there's a problem. Maybe part of
the crowbar cirucit (although I wouldn't have thought that shorting the
input would shut the outputs down fast enough), maybe something to blow
the fuse if the input votlage selector is set incorrectly (the HP
Integral PSU contains a triac circuit the sole purpose of which is to
blow the mains fuse if machine is connected to 230V mains with the
selector set to 115V).
The latter sounds like a distinct possibility.
The rectifier circuit is actually as follows, running as a rectifier /
doubler depending on the state of J1 (culled from the smpsu repair faq):
D1
AC o-----+----|>|-------+---------+-----o DC (+)
~| D2 |+ |
+----|<|----+ | +_|_
D3 | | C1 ---
+----|>|----|--+ - |
| D4 | +--o-o--+ +320 VDC to chopper
AC o---+-+----|<|----+ - | J1 |
|~ | | +_|_
+-------------|----+ C2 ---
| - |
+------------+-----o DC (-)
... except in this case C1 and C2 are actually two capacitors in
parallel rather than individuals, and J1 is under relay control (I've
checked that it isn't accidentally operating and trying to dump 600VDC
into things :-)
I think what I'd do there is connect a light bulb
in place of the
resistor (say a normal 100W mains bulb, which should be OK for testing on
light/no load), then pull the chopper transistors and power up. If the
bulb lights brightly you've probably got a short in the
rectifier/smoothing capacitor stage.
OK, with the chopper array disconnected (plus everything downstream of
it) the bulb lights very brightly when power's initially applied, then
gradually extinguishes over a period of about 5 seconds. DC output from
the rectifier is 300V. Possible dried-out smoothing cap? The bridge Now what happens if you add the choppers?
try the whole PSU with the bulb in place, or isolate everything
downstream of the choppers and just try those hooked to the DC output of
the bridge? (i.e. by removing the main switching transformer and the
small transformer that drives the gates of the choppers I should be able
to isolate everything else)
cheers
Jules